The present invention relates to signal distribution systems and to signal combiner networks employed therein.
FIG. 1A illustrates a conventional signal distribution system useful for processing satellite signals as known in the art. The system 100 employs antennas 110, 112, each operable to collect transponder signals 102a,b and 102c,d which may be grouped within a common IF-band signal (e.g., L-band). The transponder signals 102 may be communicated at various frequencies (e.g., Ku or Ka-bands) and at different polarizations (e.g., 102a,b at horizontal and vertical, and 102c,d at left-hand and right-hand circular polarizations). The system further includes an outdoor unit 120 (ODU), which may be in the form of a low noise block (LNB). The ODU 120 includes a downconverter switch 122 operable to downconvert a band of channels/transponders of a received signal 102a-102d to an IF frequency (e.g., L-band) and to switchably couple the downconverted band of channels to any one or more of the cables 1301-130n for distribution to corresponding set-top boxes (STB) 1401-140n. Each set top box 140 includes a tuner 142 for tuning to a particular channel/transponder of a particular satellite signal 102a-120d. When the tuner 1421 is controlled to receive a desired channel, the ODU 120 is configured to receive the desired satellite signal and downconverts an entire band of channels to an IF frequency, placing said band of channels on the cable 1301 for transmission to the tuner 1421. The tuner 1421 then selects the desired channel from the band of channels present on the cable 1301. Because the band downconverter switch 122 operates to supply a band of channels to each cable 1301-130n, each tuner 1401-140n receives a group of channels along its respective cable 1301-130n in order to select the particular channel the tuner wishes to receive.
FIG. 1B illustrates a signal distribution system implementing a channel stacking switch known in the art. In this system, a channel stacking switch 160 is implemented to permit multiple tuners 1401-140n to access a desired transponder/channel with a single cable 130. The channel stacking switch 160 resides in the ODU 170, the channel stacking switch 160 having multiple inputs, each input operable to receive a band of channels supplied by the downconverter switch 122. When, for example, tuner 1421 in the set-top box 1401 is controlled to receive a particular channel, downconverter switch 122 is controlled to provide a band of channels to output 122a, the supplied band of channels including the desired channel. This downconverted band of channels is supplied to the input 160a of the channel stacking switch 160, which frequency translates the downconverted band of channels, such that the desired channel is frequency translated to frequency f1 (e.g., using mixer 1621 to translate the desired channel to frequency f1), frequency f1 being previously assigned to the requesting tuner 1421. The frequency-translated band of channels is applied to a filter 1631, whereby the desired channel is frequency aligned to the passband portion of the filter 163 to pass the desired channel and to remove the unselected channels of the band of channels. Signal combiner 168 operates to combine each of the filtered signals, thereby forming a frequency-multiplexed signal output onto cable 130. Each tuner 1401-140n is configured to tune to a particular frequency of the frequency-multiplexed signal, and in this manner, each of the n different tuners can simultaneously receive its signal from the single cable 130.
FIG. 1C illustrates a signal combiner network implemented in the channel stacking switch 160 known in the prior art. As previously described, filters 163 operate to remove unwanted portions of the signal spectrum that are supplied to a signal combiner 168. Additionally, variable gain amplifiers 164 may be implemented to provide a signal leveling function, whereby each of the filtered signals supplied to the signal combiner 168 are substantially the same amplitude.
While the aforementioned system is generally effective in providing a means for supplying multiple transponders/channels along a single cable to multiple output devices/tuners, some drawbacks are present. In particular, insertion loss of the signal combiner 168 can be high, especially for combiners having a large number of input ports. To overcome this loss, higher gain of amplifiers 164 and/or additional amplifiers may be required. This comes at the cost of possibly higher spurious products (and power consumption generated thereby), because each amplifier generates broadband noise at the output (and possibly distortion terms), falling on and adversely affecting other channels after combining in 168, thus reducing the efficacy of the filtering performed in the preceding stages 163.
Therefore there is room in the art for improvement of the system performance by reducing the adverse effect of the broadband amplifier noise, addressed hereinafter by the present invention.